This invention pertains generally to internal combustion engine control systems, and more specifically to control of a variable valve system.
Engine manufacturers incorporate variable valve systems, including variable cam phasing systems, to improve operating and emissions performance of internal combustion engines. Distinct engine operating characteristics resulting from use of the variable valve system include improved combustion stability at idle, improved airflow into the engine over a range of engine operations corresponding to improvements in engine performance, and improved dilution tolerance in a combustion charge. Benefits of incorporating the variable valve system into an engine include improved fuel economy, improved torque at low engine speeds, lower engine cost and improved quality through elimination of external exhaust gas recirculation (EGR) systems, and improved control of engine exhaust emissions.
A typical internal combustion engine is comprised of at least one cylinder containing a piston that is attached to a rotating crankshaft by a piston rod. The piston slides up and down the cylinder in response to combustion events that occur in a combustion chamber formed in the cylinder between the piston and a head. The head contains one or more intake valves to control the flow of air and fuel into the combustion chamber, and one or more exhaust valves that control the flow of exhaust gases out of the combustion chamber. A rotating camshaft opens and closes the intake and exhaust valves, and is synchronized with the position of each piston and the crankshaft. As an example of a variable valve system, a typical variable cam phasing system a variable cam phaser attached to an engine camshaft, and a cam position sensor that measures rotational position of the camshaft. The variable cam phasing system varies the opening and closing of each valve by varying angular position and rotation of the camshaft, relative to angular position and rotation of the crankshaft and each respective cylinder. An oil control valve diverts flow of pressurized engine oil to control the variable cam phaser, primarily based upon feedback from the cam position sensor. Typically an electronic engine controller controls this operation.
Engine oil contained in the variable valve system drains into an engine crankcase subsequent to engine shutdown. The rate of drainage from the variable valve system and time necessary to completely drain the system is not readily determinable. Therefore, the amount of oil left in the variable valve system at engine restart is unknown. When the engine is restarted, the control system for the variable valve system may immediately attempt to control position of the valve system, to achieve driveability and emissions benefits resulting from operation of the variable valve system. When there is insufficient oil to operate the system, the result is unstable engine operation. This includes the variable valve device impacting against an engaged locking pin, causing audible noise and wear of the variable valve device. If the locking pin is disengaged, the variable valve device may be uncontrolled, thus affecting engine performance when the variable valve device is not able to attain the desired control position. Engine performance is adversely affected until a sufficient quantity of oil is pumped into the variable valve system to enable effective control of the variable valve system.
Engine performance is affected because control of the valve opening affects mass of air flowing into an individual cylinder, thus affecting volumetric efficiency of the internal combustion engine. This in turn affects quantity of fuel delivery, because fuel delivery is typically determined by measuring or calculating mass air flow and determining an air/fuel ratio that is required to meet operator performance requirements and engine emissions requirements. The quantity of fuel delivered to each cylinder is determined based upon the mass airflow and the required air/fuel ratio. A combustion charge is created in each combustion chamber by delivering the quantity of fuel near the intake valve of the cylinder, or directly into the cylinder. This is known to one skilled in the art. When the mass air flow into the cylinder is unpredictable, due to an unknown position of the variable cam phaser, the controller may overfuel or underfuel the combustion charge. This results in problems with combustion stability and variations in air/fuel ratio that affect emissions, engine noise, and driveability.
Pressure and flow of engine oil into the variable valve system is affected by several factors in the system at engine start and initial operation. These factors include engine oil pump capacity; oil temperature, viscosity, age and level of contamination; variable valve system part-to-part variability, caused by manufacturing tolerances and component wear; and engine temperature at startup. These factors result in an inability of the controller to precisely determine position of the variable valve device. The previously described benefits derived from use of a variable valve system may be compromised due to the variations. An engine with dual cylinder banks may experience differences between the two banks that are caused by differences in oil pressure and flow at each bank. The result is further reduced engine performance during engine start and initial operation due to vibration and engine instability caused by variations in bank-to-bank airflow, individual cylinder fueling, and volumetric efficiencies.
The prior art has sought to eliminate the problem of oil drainage from the engine and the variable valve system by delaying operation of the variable valve system for a predetermined amount of time subsequent to an engine start event, to allow buildup of engine oil pressure. This delayed operation may result in driveability complaints and increases in engine emissions if operation is delayed for a significant amount of time. What is needed is a system that operates the variable valve system immediately after engine start, and disengages the variable valve system when engine performance is unacceptable.
The present invention provides an improvement over conventional engine controls by providing a method and system that operates a variable valve system immediately after engine start, and disengages the variable valve system when engine performance is unacceptable. If the variable valve system is disengaged after engine start due to poor engine performance, a time delay occurs to allow the engine to create a sufficient amount of oil pressure to operate the variable valve system.
The invention includes a method to control a variable valve device for an internal combustion engine. This includes operating the variable valve device immediately subsequent to an engine start event, and monitoring a first control error and discontinuing the operation of the variable valve device if the first control error is greater than a first calibratable value. The first control error comprises calculating a standard error based upon a difference between a measured position of the variable valve device and a commanded position of the variable valve device. This is calculated over a predetermined amount of time subsequent to the engine start event. The method further comprises monitoring a second control error and discontinuing the operation of the variable valve device if the second control error is greater than a second calibratable value. The second control error comprises the integral term of a predetermined proportional-integral-derivative control strategy. The variable valve device comprises a variable cam phaser, or a variable lift and duration device, or a variable valve timing device.
The invention also includes a system to control the variable valve device for an internal combustion engine, including a controller comprised of internal algorithms and calibrations, and operable to control the variable valve device. The controller is electrically connected to at least one sensor that monitors engine operating conditions. The controller executes the internal algorithms and calibrations to operate the variable valve device immediately after an engine start event, and monitor engine operating conditions. The controller determines a first control error and a second control error, based upon the monitored engine operating conditions, and enables the variable valve device to operate only if the first control error is less than a first calibratable value and the second control error is less than a second calibratable value.
These and other aspects of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description of the embodiments.